Abstract

Phosphate removal is an important measure to control eutrophication in aquatic environments, as it inhibits algal bloom. Salinity exists in these media along with high phosphate and currently available phosphate removal methods function poorly under this condition. In this study, the main objective is to fabricate a nanocomposite to improve and accelerate phosphate removal from saline solutions. To achieve this goal, Fe3O4/ZnO and a novel nanoadsorbent, Fe3O4/ZnO/CuO, were synthesized. Their characteristics were determined using FE-SEM, EDX, FT-IR, and XRD analyses, and their capability to adsorb phosphate from saline solutions was investigated and compared. The overall results suggest that the trimetallic oxide nanocomposite has great potential for the efficient removal of phosphate, in comparison with Fe3O4/ZnO. Experiments showed that Fe3O4/ZnO/CuO exhibited a remarkable sorption capacity of 156.35 mg P/g, fast sorption kinetic, strong selectivity for phosphate even in the presence of a high concentration of salinity (60 mg/L), and a wide applicable pH range of 3–6. Furthermore, using Fe3O4/ZnO/CuO, even a low dosage of 0.1 g/L was sufficient to reach an adsorption efficiency of 96.13% within 15 min compared to Fe3O4/ZnO (80.47% within 30 min). Moreover, the pseudo-second-order kinetic model best described the experimental adsorption data for both nanocomposites.

HIGHLIGHTS

  • High adsorption capacity of the trimetallic oxide nanocomposite.

  • Wide applicable pH range of 3–6 of Fe3O4/ZnO/CuO nanocomposite.

  • Enhanced removal from saline solution by Fe3O4/ZnO/CuO.

  • More than 45% adsorption rate increase in saline solution by modifying Fe3O4/ZnO.

  • The pseudo-second-order kinetic model perfectly described removal.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

Phosphorus (P), as an indispensable element, holds a vital nourishing role in the growth of all living organisms (Shan et al. 2020). However, their excessive presence in the environment originating from industries, agricultural, and domestic wastewater sources could accelerate the overgrowth of undesirable algae and aquatic plants. This algae bloom leads to the eutrophication phenomenon, an important issue that damages the biodiversity and functionality of the aquatic ecosystem (Wu et al. 2017; Li et al. 2018; Ma et al. 2020). Therefore, controlling the amount of phosphorus is of great importance.

High amounts of salt are used in oil, textile, and leather industries. During their processes, a large amount of saline wastewater is produced, which finally enters surface waters. Moreover, high amounts of NaCl and Na2SO4 salts exist naturally in free waters as well (Stewart 2008). According to the results of various studies, the phosphorous removal efficiency decreases significantly with an increase in the amount of salinity in the solution (Lefebvre & Moletta 2006). Therefore, it is of high significance to find a method that functions well under this condition.

Thus far, conventional approaches such as chemical precipitation and biological treatment are incapable of removing low concentrations of phosphate; therefore, other methods are required so as to overcome the mentioned problems (Wu et al. 2017). Due to the simplicity in design, the effectiveness even at low concentrations of phosphorus, and the recycling capability, the adsorption method has been widely accepted as a preferable approach for phosphate removal (Wu et al. 2017; Yin et al. 2018; Dong et al. 2020). Since conventional adsorbents show low selectivity for phosphate adsorption in the presence of other competing anions (such as sulfate, chloride, and bicarbonate), the identification and development of adsorbents with high selectivity for this nutrient are crucial (Acelas et al. 2015). Recently, therefore, nano-based adsorbents have been developed due to their ease of use, low cost, and contamination removal capability from water and wastewater. So far, various nanocomposites have been used for phosphate adsorption. For example, Nie et al. (2019) achieved 44.14 mg P/g adsorption capacity by using Ti–NS nanocomposite at pH 6.5–7.5, nanocomposite dosage of 500 mg/L, initial phosphate concentration of 10 mg/L, and at 25 °C. In Zhou et al.’s (2018) study, the adsorption capacity of 37.86 mg P/g was reported by the use of nHFZO@I402 nanocomposite at pH 7, nanocomposite dosage of 750 mg/L, phosphate concentration of 5 mg/L, and at 25 °C. In another study, Wu et al. (2017) achieved the adsorption capacity of 83.5 mg P/g by using La(OH)3/Fe3O4 nanocomposite at pH 4.3–6, nanocomposite dosage of 100 mg/L, initial phosphate concentration of 0.5–15 mg/L, and at 23 °C. In Nodeh et al.’s (2017) research, the adsorption capacity of 116.28 mg P/g was achieved by using MG@La nanocomposite at pH 6, nanocomposite dosage of 100 mg/L, initial phosphate concentration of 50 mg/L, and at 25 °C.

Moreover, the recyclability of an adsorbent is of great importance to evaluate its performance for practical applications (Hu et al. 2016). Nanoparticles are difficult to separate after usage in wastewater treatment, and commonly used adsorbent recycling systems suffer from several disadvantages. While separation methods such as centrifuge consume a lot of energy, and filtration is susceptible to clogging, magnetic separation is a faster and more effective method (Wu et al. 2017; Cai et al. 2019). Accordingly, the combination of magnetic nanoparticles (such as Fe3O4) with other nanomaterials makes the separation and recycling of the synthesized adsorbent more facile.

Earlier research conducted by Nezhadheydari et al. (2019) showed that Fe3O4/ZnO nanocomposite has a phosphate removal efficiency of 50% in 12 h, requiring modification. For this purpose, CuO nanoparticles were used because of their cost-efficiency, simple synthesis, and the fact that the combination of ZnO and CuO creates environmentally friendly nanocomposites. Also, through the formation of inner-sphere and/or outer-sphere complexes, CuO nanoparticles show a strong ligand sorption (of ) (Mahdavi & Akhzari 2016).

However, to the best of our knowledge, there are no reports available on the application of Fe3O4/ZnO/CuO for phosphate removal. In addition, the functionality of this trimetallic oxide nanocomposite for phosphate adsorption remains unclear and requires more investigations.

Motivated by these concepts, the main objectives of this study are to (1) fabricate and characterize a novel recyclable trimetallic oxide nanocomposite for phosphate removal, (2) improve the phosphate sorption efficiency and the capacity of the trimetallic oxide adsorbent, (3) accelerate phosphate removal by modifying Fe3O4/ZnO, and (4) investigate the kinetic studies of phosphate removal in the presence and absence of salinity by Fe3O4/ZnO/CuO. This study proposed a novel adsorbent as a promising candidate for effective phosphate removal from saline solutions.

MATERIAL, EQUIPMENT, AND METHODS

Material

In this study, all chemicals were of analytical grade and used without further purification. Iron(III) chloride hexahydrate (FeCl3·6H2O), zinc acetate dihydrate (Zn(CH3COO)22H2O), ethylene glycol, polyethylene glycol, sodium acetate (C2H3NaO2), copper(II) acetate dihydrate (Cu(CO2CH3)2 · 2H2O), sodium hydroxide (NaOH), ethanol and distilled water were used for synthesizing the nanocomposites. KH2PO4 was used as a pollutant, and NaCl and Na2SO4 were used for investigating the effect of salinity on phosphate adsorption. Molybdovanadate reagent was also used for analyzing the concentration of phosphate in the solution, all of which were purchased from Merck.

Equipment

The equipment used in this research was Hach DR4000U spectrophotometer, Metrohm 691 pH meter, Korea Tech DSA-series ultrasonic bath, Shafaq magnetic stirrer (made in Iran), Fara Azma vacuum oven (made in Iran), 101 Sigma centrifuge, Stainless Teflon autoclave (Made in Iran), Tescan MIRA3 (for FE-SEM and EDX analysis), XRD Philips X'pert XPD, and Frontier FT-IR (made in the USA).

Synthesis of nanocomposites

To synthesize the nanoadsorbents, the following steps were taken for each of the nanocomposites.

Preparation of Fe3O4/ZnO

Fe3O4 and ZnO nanoparticles were synthesized by co-precipitation and sol-gel methods, respectively (Fu & Zhu 2016; Hasnidawani et al. 2016). To prepare the nanoparticles, 1 g of FeCl3·6H2O was poured into 40 ml of ethylene glycol and stirred to obtain a clear solution. Then, 4.3 g of sodium acetate and 1 g of polyethylene glycol were added to the solution, stirred at room temperature for 30 min, and put in an autoclave for 8 h at 200 °C. Then, it was washed several times with ethanol and dried at 60 °C in the vacuum oven. Then, 0.5 g of the dried compound was dispersed in 100 ml of ethanol. Two solutions were prepared: one containing 2 g of Zn(CH3COO)2·2H2O in 15 ml of distilled water and the other containing 8 g of sodium hydroxide in 10 ml of distilled water; both of which were stirred at room temperature for 5 min. The resulting solution was added drop-wise to a mixture of Fe3O4 and ethanol. The final solution was kept at room temperature for 1 h to allow all sediments to settle. Then, it was centrifuged several times with ethanol and was dried at room temperature.

Preparation of Fe3O4/ZnO/CuO

CuO nanoparticles were synthesized using the hydrolysis method (Tran & Nguyen 2014). To prepare these nanoparticles, 0.6 g of copper acetate (II) was dissolved in 50 ml of distilled water on a stirrer at 100 °C, and 0.01 g of NaOH was added. Then, 0.1 g of the synthesized Fe3O4/ZnO nanocomposite was dispersed in 25 ml of distilled water, added to the copper solution, and stirred for 1 h at 80 °C. After hydrolysis, the precipitate in the solution was washed several times with distilled water and ethanol, and then the resulting substance was dried at 200 °C for 2 h.

Methods

Phosphate adsorption from the synthesized wastewater using Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites was investigated by the one-factor-at-a-time (OFAT) method. The values of the examined parameters are presented in Table 1.

Table 1

Parameters

ParametersValues
pH 2, 3, 4, 5, 6, 7, 9, and 11 
Initial concentration of nanocomposite (mg/L) 20, 60, 100, 140, 180, and 200 
Initial concentration of phosphate (mg/L) 10, 20, 40, 50, 60, and 100 
Concentrations of NaCl and Na2SO4 salts (mg/L) 10, 20, 30, 40, 50, and 60 
ParametersValues
pH 2, 3, 4, 5, 6, 7, 9, and 11 
Initial concentration of nanocomposite (mg/L) 20, 60, 100, 140, 180, and 200 
Initial concentration of phosphate (mg/L) 10, 20, 40, 50, 60, and 100 
Concentrations of NaCl and Na2SO4 salts (mg/L) 10, 20, 30, 40, 50, and 60 
A 100 ml glass beaker was used for phosphate removal examinations at a temperature of 25 ± 0.5 °C, and all experiments were performed three times, the error rate of which was less than 1.5%. After each test, the used nanocomposite was separated with a magnet to avoid a possible metal leakage. The remaining liquid in the beaker was used to determine the concentration of residual phosphate in the solution by the colorimetric method using the UV–Vis spectrophotometer and the molybdovanadate reagents (0.5 ml) (Eaton et al. 2005). Then, the phosphate adsorption efficiency was calculated using the following equation:
formula
(1)
where C0 (mg/L) is the initial concentration of the phosphate solution, Ce (mg/L) is the concentration of phosphate solution after adsorption, and E (%) is the efficiency of phosphate adsorption.
The capacity of phosphate adsorption by Fe3O4/ZnO and Fe3O4/ZnO/CuO adsorbents was calculated using the following equation (Lin et al. 2020):
formula
(2)
where C0 and Ce (mg/L) are initial and equilibrium concentrations of the phosphate solution, respectively, V (L) is the volume of the phosphate solution, and Ms (g) is the mass of the utilized nanocomposite.

After obtaining the optimum conditions for phosphate removal, phosphate adsorption tests were conducted in the presence of NaCl and Na2SO4 salts, which are the predominant salts in both industrial wastewater and free waters (Stewart 2008). Then, the amount of adsorption efficiency was determined.

Adsorption kinetics were also investigated by pseudo-first-order and pseudo-second-order kinetic equations (Equations (3) and (4)) (Ho & McKay 1998):
formula
(3)
formula
(4)
where K1 (L/h) is the rate constant of the pseudo-first-order model, K2 (g/mg h) is the rate constant of the pseudo-second-order model, and q and qe (mg/g) are the amount of phosphate adsorbed to the adsorbent at time t (min).

To determine pH at the point of zero charge (pHpzc) for each nanocomposite, the following approach was used. First, 0.01 M NaCl solution was added to sealed vials, and the initial pH of each vial was adjusted to the values between 2 and 10 using pH regulator solution. 30 mg of each nanocomposite was added to vials with different initial pH and shaken for 48 h at room temperature. Using a pH meter, the final pH of each solution was determined. The initial and final graphs were plotted, and pHpzc is the interception point of the pHfinal vs. pHinitial curve and the pHinitial = pHfinal line (Órfão et al. 2006).

RESULTS AND DISCUSSION

Magnetic adsorbent characteristics

To determine the characteristics of the synthesized nanocomposites, FE-SEM, EDX, FT-IR, and XRD analyses were performed, the results of which are presented in Figures 14.

Figure 1

FE-SEM analyses of (a) Fe3O4/ZnO morphology and dimensions and (b) Fe3O4/ZnO/CuO morphology and dimensions.

Figure 1

FE-SEM analyses of (a) Fe3O4/ZnO morphology and dimensions and (b) Fe3O4/ZnO/CuO morphology and dimensions.

Figure 2

EDS spectra of (a) Fe3O4/ZnO nanocomposite and (b) Fe3O4/ZnO/CuO nanocomposite.

Figure 2

EDS spectra of (a) Fe3O4/ZnO nanocomposite and (b) Fe3O4/ZnO/CuO nanocomposite.

Figure 3

FT-IR spectra of (a) Fe3O4/ZnO nanocomposite and (b) Fe3O4/ZnO/CuO nanocomposite.

Figure 3

FT-IR spectra of (a) Fe3O4/ZnO nanocomposite and (b) Fe3O4/ZnO/CuO nanocomposite.

Figure 4

XRD patterns of (a) Fe3O4/ZnO/CuO nanocomposite and (b) Fe3O4/ZnO/ nanocomposite.

Figure 4

XRD patterns of (a) Fe3O4/ZnO/CuO nanocomposite and (b) Fe3O4/ZnO/ nanocomposite.

Figure 5

(a) Effect of pH on phosphate adsorption efficiency in optimum condition and (b) effect of pH on phosphate adsorption efficiency over time ([Fe3O4/ZnO] = 100 mg/L, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C). (c) pHpzc of Fe3O4/ZnO and (d) pHpzc of Fe3O4/ZnO/CuO.

Figure 5

(a) Effect of pH on phosphate adsorption efficiency in optimum condition and (b) effect of pH on phosphate adsorption efficiency over time ([Fe3O4/ZnO] = 100 mg/L, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C). (c) pHpzc of Fe3O4/ZnO and (d) pHpzc of Fe3O4/ZnO/CuO.

FE-SEM analysis

To investigate the morphology and dimensions of Fe3O4/ZnO and Fe3O4/ZnO/CuO, FE-SEM images of both fabricated nanocomposites are presented in Figure 1. Figure 1(a) shows the morphology of Fe3O4/ZnO, which has a rod-shaped structure, and the dimension of this nanocomposite varies from 20 to 30 nm. In Hong et al.’s (2008) and Xia et al.’s (2011) studies, the average dimension of Fe3O4/ZnO nanocomposite was in the ranges of 25–30 and 20–30 nm, respectively. As can be seen in Figure 1(b), Fe3O4/ZnO/CuO has a spherical structure, and the dimension of this nanocomposite varies in the range of 25–36 nm.

EDX analysis

X-ray diffraction analysis was performed to identify the chemical composition of the synthesized samples. X-ray diffraction spectra for Fe3O4/ZnO and Fe3O4/ZnO/CuO are shown in Figure 2(a) and 2(b), respectively. The constituent materials of the two synthesized nanocomposites are presented in Table 2. Based on results, Fe3O4/ZnO has O, Fe, and Zn elements, and the trimetallic oxide adsorbent contains O, Fe, Zn, and Cu. According to Figure 2(a) and 2(b), broad peaks of Zn, O, Fe, and Cu elements indicate a strong bond between Fe3O4 and ZnO nanoparticles in Fe3O4/ZnO and between Fe3O4, ZnO, and CuO nanoparticles in the Fe3O4/ZnO/CuO nanocomposite (Farrokhi et al. 2014; Tju et al. 2017).

Table 2

Weight percentage of the constituent elements of nanocomposites

NanocompositeWeight percentage (%)
OFeZnCuCumulative percentage
Fe3O4/ZnO 44.96 45.99 9.05 100 
Fe3O4/ZnO/CuO 42.25 44.31 9.01 4.43 100 
NanocompositeWeight percentage (%)
OFeZnCuCumulative percentage
Fe3O4/ZnO 44.96 45.99 9.05 100 
Fe3O4/ZnO/CuO 42.25 44.31 9.01 4.43 100 

FT-IR analysis

FT-IR was used to qualify the chemical bonds between the functional groups of Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites. The synthesized samples were subjected to wavenumbers in the range of 400–4,000 cm−1, the results of which are presented in Figure 3. According to Figure 3(a), the adsorption in 400 and 590 cm−1 are associated with stretching bands of Zn–O and Fe–O, respectively. Moreover, the broad peak around 1,700–3,400 cm−1 is related to the stretching vibrations of O–H bond in water molecules adsorbed on the surface of Fe3O4/ZnO nanocomposite. As shown in Figure 3(b), the adsorption in 400, 420, and 590 cm−1 are related to stretching bands of Zn–O, Cu–O, and Fe–O, respectively. Besides, the wide peak in 1,700–3,400 cm−1 is due to the stretching vibrations of O–H bond in water molecules adsorbed on the surface of Fe3O4/ZnO/CuO (Xie et al. 2015; Kulkarni et al. 2017; Tju et al. 2017). The agreement of XRD and EDX analyses indicates the successful formation of fabricated nanocomposites (Taufik & Saleh 2017).

XRD analysis

XRD patterns were generated using Cu Kα radiation (40 kV, 40 mA) to identify the crystalline components of both adsorbents. All XRD patterns were obtained over a 2ϴ range of 5–80°. Figure 4(a) and 4(b) illustrates XRD patterns of Fe3O4/ZnO and Fe3O4/ZnO/CuO nanoadsorbents, respectively. Fe3O4 nanoparticles have diffraction angles at about 2ϴ = 30, 35, 43, 53, 57, 62, 66, 71, 74, 75, and 79, ZnO nanoparticles at 2ϴ = 32, 34, 36, 47, 56, 63, 66, 68, and 69 (Farrokhi et al. 2014; Vakili Tajareh et al. 2019), and CuO particles have diffraction angles at 2ϴ = 32, 35, 38, 48, 53, 58, 61, 65, 66, and 67 (Taufik et al. 2016).

Determining the optimal condition of the two nanoadsorbents for phosphate removal

To determine the optimum values of parameters (the initial pH of the solution, the concentration of nanocomposite, and the initial concentration of phosphate), required examinations were conducted. Then, the optimum conditions for each of the desired parameters were determined. The effect of different dosages of salinity and adsorption kinetics under optimum conditions were also investigated.

Effect of pH

Both the dominant phosphate species and surface charges of a nanocomposite in water are highly dependent on pH. Therefore, experiments were conducted to understand the effect of solution pH on the phosphate removal efficiency with the use of both adsorbents. Using pH regulator solution, all phosphate solutions were adjusted to a wide pH range of 2–11, the results of which are displayed in Figure 5. As can be seen in Figure 5(a), the acidity or alkalinity of the solution had a significant effect on the phosphate removal efficiency. Experimental data showed that the efficiency of the process was increased sharply and then decreased to further increase of pH. According to the results, both nanocomposites had a better performance in the acidic range (Fe3O4/ZnO at pH = 3 and Fe3O4/ZnO/CuO at pH = 4). As known, the phosphate in the solution changes into distinct types at different pH ranges, as shown in the following equation:
formula
(5)
where pK1 = 2.12, pK2 = 7.21, and pK3 = 12.67, respectively (Hong et al. 2017; Othman et al. 2018). pHpzc was calculated for both adsorbents. As shown in Figure 5(c) and 5(d), pHpzc of Fe3O4/ZnO and Fe3O4/ZnO/CuO are 6 and 6.76, respectively. At lower pH (under pHpzc), due to protonation, both nanocomposite surfaces were positively charged, and the dominant phosphate ions were more easily adsorbed on the adsorbents' surface. Therefore, the adsorption efficiency was higher in acidic pH. As pH increases (more than pHpzc), through deprotonation, the positively charged surface of both of the nanocomposites gradually changes into negative. As a result, the adsorption efficiency dropped due to the negligible attraction between the dominant anion species and the negative charges on the adsorbents’ surface (Nezamzadeh-Ejhieh & Hushmandrad 2010; Farrokhi et al. 2014; Taufik & Saleh 2017; Karthikeyan & Meenakshi 2020; Salehi & Hosseinifard 2020). In many studies, a similar tendency was indicated regarding the removal of phosphate (Liu et al. 2011; Eljamal et al. 2016; Hong et al. 2017; Salehi & Hosseinifard 2020).

Since the optimum pH value of the trimetallic oxide nanocomposite (pH = 4) is higher than that of Fe3O4/ZnO (pH = 3), the amount of material needed to reduce pH will decrease. Moreover, Fe3O4/ZnO/CuO has an efficiency of 96.13% at pH = 4, with an increase of nearly 16% compared to Fe3O4/ZnO with a removal efficiency of 80.46% at pH = 3. Furthermore, at pH in the wide range of 3–6, the trimetallic oxide adsorbent is highly efficient. These results show a significant improvement in the quality of Fe3O4/ZnO/CuO and its more economical use.

The effect of the initial concentration of nanocomposite

To investigate the effect of the adsorbents dosage, different amounts of them were tested, the results of which are presented in Figure 6. As can be observed in Figure 6(a), at first, the adsorption efficiency rises with an increase in the concentration of both nanocomposites and decreases slightly after reaching the maximum value. This trend can be ascribed to an increase in adsorbent dosage, which leads to the lack of access to the unsaturated active sites in the adsorbents (Farrokhi et al. 2014; Salehi & Hosseinifard 2020). Also, when the adsorbent dosage exceeds 100 mg/L, the particles become agglomerated, which culminates in a decrease in surface area, and thus a reduction in the adsorption efficiency (Keramati & Ayati 2019). Another important point is that the value of adsorption capacity increased while using a solution that contains 100 mg/L of the trimetallic oxide nanocomposite (156.35 mg/g) in comparison with the same amount of Fe3O4/ZnO (130.76 mg P/g). This increment can be attributed to the CuO nanoparticles. Since these particles exhibit a strong ligand sorption (of ) through the formation of inner-sphere and/or outer-sphere complexes, the phosphate adsorption efficiency and capacity increase significantly (Mahdavi & Akhzari 2016).

Figure 6

(a) Effect of nanocomposite amount on phosphate absorption efficiency in optimum conditions and (b) effect of Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites on the phosphate uptake efficiency over time (pH = 3, [Fe3O4/ZnO] = 20–200 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 20–200 mg/L, , and T = 25 ± 0.5 °C).

Figure 6

(a) Effect of nanocomposite amount on phosphate absorption efficiency in optimum conditions and (b) effect of Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites on the phosphate uptake efficiency over time (pH = 3, [Fe3O4/ZnO] = 20–200 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 20–200 mg/L, , and T = 25 ± 0.5 °C).

Effect of initial phosphate concentration

To determine the optimum phosphate concentration, different amounts of this contaminant were examined, the results of which are presented in Figure 7. Based on the results, by increasing the initial concentration of phosphate, the adsorption rate raised at first and then decreased. This trend could be related to the presence of sufficient active sites in the adsorbent compared to the initial concentration of the contaminant. By increasing the concentration to 50 mg/L, the adsorption rate for Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites reached 80.46% and 96.31%, respectively. Then the adsorption efficiency, in concentrations of more than 50 mg/L, decreased due to the reduction in the number of active sites in comparison to the dosage of contaminant (Hallaji et al. 2015; Nakarmi et al. 2020). In several studies, increasing the initial phosphate concentration has been associated with a decrease in the phosphate removal efficiency (Bozorgpour et al. 2016; Nodeh et al. 2017). Besides, by comparing the graphs in Figure 7(a), it can be concluded that the trimetallic oxide nanocomposite can maintain its high adsorption efficiency even at a low concentration of phosphate (10 mg/L) in comparison to Fe3O4/ZnO.

Figure 7

(a) Effect of initial phosphate concentration on its adsorption efficiency under optimum conditions and (b) effect of initial phosphate concentration on its adsorption efficiency by Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites over time (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.55 °C).

Figure 7

(a) Effect of initial phosphate concentration on its adsorption efficiency under optimum conditions and (b) effect of initial phosphate concentration on its adsorption efficiency by Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites over time (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.55 °C).

Equilibrium time

Phosphate adsorption for both nanocomposites was tested in 1 h. Considering the graphs illustrated in Figures 5(b), 6(b), and 7(b), the equilibrium time for Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites was attained in 30 and 15 min, respectively. This noticeable decrease in the equilibrium time indicates a significant improvement of the trimetallic oxide adsorbent in comparison to Fe3O4/ZnO due to the reduction in the consumed energy during the process.

The effect of salinity on the phosphate removal efficiency

To investigate the ability of the synthesized nanocomposites in phosphate removal in the presence of salinity, the influence of different anions was investigated. The effect of chloride and sulfate anions, as a predominant ion that generally coexists with phosphate in both industrial wastewater and free waters, was examined. These ions were tested at concentrations of 10, 20, 30, 40, 50, and 60 mg/L, and their results are presented in Figure 8(a) and 8(b) for Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites, respectively. As can be seen, the effect of both salts on the phosphate removal efficiency was almost constant. The presence of both ions had an interfering effect on the phosphate adsorption and led to a decrease in the adsorption efficiency. The investigated anions' chemical attraction to the adsorption sites can be explained by the Hofmeister order, i.e., (Luo et al. 2020). However, salinity had less effect on the trimetallic oxide adsorbent in comparison to Fe3O4/ZnO. Also, Fe3O4/ZnO/CuO had a higher adsorption capability even at high salinity concentrations (60 mg/L) (less than 16% decrease in efficiency). On the other hand, Fe3O4/ZnO had a significant efficiency reduction in the presence of salinity (more than 36%). Moreover, Fe3O4/ZnO/CuO achieved higher efficiency in less time, indicating its higher ability for phosphate removal. According to the results, the phosphate removal efficiency in the presence of the mentioned concentrations of NaCl salinity using Fe3O4/ZnO was 70.95, 66.12, 56.31, 57.4, 48.6, and 39.81%, respectively, while this efficiency for trimetallic oxide nanocomposite was 90.74, 89.32, 88.02, 85.62, 83.45, and 80.37%, respectively. The phosphate removal efficiency in the presence of the mentioned concentrations of Na2SO4 salinity while using Fe3O4/ZnO was 76.4, 75.3, 55.3, 53.9, 49.5, and 43.72%, respectively, and this efficiency using Fe3O4/ZnO/CuO was 90.13, 88.48, 87.23, 86.33, 83.69, and 81.05%, respectively. The decrease in the contaminant removal efficiency in the presence of salinity can be due to the competition that occurs between ions. Similar results have been reported in several studies. In Hong et al.’s (2017) study, Fe3O4@SiO2 nanocomposite was used in the presence of anion, which reduced the phosphate removal efficiency by 29%. In a study by Hu et al. (2020), Zr@MCS nanoadsorbent was used for phosphate removal in the presence of anion, which reduced the adsorption efficiency by more than 15%.

Figure 8

Investigating the effect of salinity on the phosphate removal efficiency by synthesized nanocomposites: (a) Fe3O4/ZnO and (b) Fe3O4/ZnO/CuO (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C).

Figure 8

Investigating the effect of salinity on the phosphate removal efficiency by synthesized nanocomposites: (a) Fe3O4/ZnO and (b) Fe3O4/ZnO/CuO (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C).

Equilibrium and kinetic adsorption studies

To evaluate the rate of phosphate adsorption on the surface of the nanocomposites, pseudo-first-order and pseudo-second-order kinetic models were applied to the adsorption data. Their results under two situations – the presence and absence of salinity – are presented in Table 3. The pseudo-second-order kinetics has a higher correlation coefficient (R2) than the pseudo-first-order (closer to 1) in the absence of salinity in the system. So, it can be concluded that phosphate adsorption kinetics follows the pseudo-second-order model. Different types of adsorbents had similar results (Bozorgpour et al. 2016; Nodeh et al. 2017; Wu et al. 2017; Zhou et al. 2018; Hao et al. 2019; He et al. 2020). According to the results, the rate constant in the kinetic equation for Fe3O4/ZnO and Fe3O4/ZnO/CuO is 0.001 and 0.0015, respectively. The 50% increase in the adsorption rate while using Fe3O4/ZnO/CuO indicates its higher capability in phosphate adsorption. Moreover, the Fe3O4/ZnO/CuO maximum adsorption capacity (156.35 mg P/g) is approximately 16.37% higher than that of Fe3O4/ZnO (130.76 mg P/g).

Table 3

Kinetics parameters for phosphate adsorption by Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites

NanocompositeType of salinityPseudo-first-order model
Pseudo-second-order model
qe (mg P/g)
K1R2K2R2
Without salinity Fe3O4/ZnO – 0.1924 0.7734 0.001 0.8426 130.76 
Fe3O4/ZnO/CuO – −0.4054 0.8614 0.0015 0.999 156.35 
With salinity Fe3O4/ZnO NaCl (60 mg/L) −0.1753 0.9017 0.0007 0.9618 64.91 
Na2SO4 (60 mg/L) 0.1647 0.8122 0.0005 0.939 71.29 
Fe3O4/ZnO/CuO NaCl (60 mg/L) −0.4311 0.7289 −0.0013 0.7468 131.1 
Na2SO4 (60 mg/L) −0.3745 0.7392 −0.0009 0.8684 132.17 
NanocompositeType of salinityPseudo-first-order model
Pseudo-second-order model
qe (mg P/g)
K1R2K2R2
Without salinity Fe3O4/ZnO – 0.1924 0.7734 0.001 0.8426 130.76 
Fe3O4/ZnO/CuO – −0.4054 0.8614 0.0015 0.999 156.35 
With salinity Fe3O4/ZnO NaCl (60 mg/L) −0.1753 0.9017 0.0007 0.9618 64.91 
Na2SO4 (60 mg/L) 0.1647 0.8122 0.0005 0.939 71.29 
Fe3O4/ZnO/CuO NaCl (60 mg/L) −0.4311 0.7289 −0.0013 0.7468 131.1 
Na2SO4 (60 mg/L) −0.3745 0.7392 −0.0009 0.8684 132.17 

Adsorption kinetics was also applied on phosphate removal in the presence of salinity (NaCl and Na2SO4 salts with a concentration of 60 mg/L) in the system. The concentration of 60 mg/L was chosen due to its substantial effect on the reduction of phosphate removal efficiency, the results of which are presented in Table 3. By comparing the values of the correlation coefficient (R2), it can be seen that the phosphate adsorption in the presence of salinity follows the pseudo-second-order model.

According to Table 3, the adsorption rate by using Fe3O4/ZnO and Fe3O4/ZnO/CuO nanocomposites in the presence of NaCl in the system (K2 = 0.0007 and K2 = 0.0013, respectively) is approximately 28 and 30%, respectively. This rate is higher than the condition that Na2SO4 exists in the system (K2 = 0.0005 and K2 = 0.0009, respectively), which shows that the interfering effect of NaCl on the adsorption efficiency is slightly less than Na2SO4. According to the results, Fe3O4/ZnO/CuO had an adsorption rate of about 45% higher than Fe3O4/ZnO. Also, using Fe3O4/ZnO and Fe3O4/ZnO/CuO adsorbents in the presence of NaCl in the system reduced the reaction rate by 30 and 13%, respectively, and by 50 and 40% in the presence of Na2SO4, respectively, compared to the absence of salinity in the system. Generally, the results indicate the better quality and performance of the modified trimetallic oxide nanocomposite.

REGENERATION AND REUSABILITY OF THE BIMETALLIC AND TRIMETALLIC OXIDE NANOCOMPOSITES

To investigate the reusability of both nanocomposites, the sorption–desorption experiments were repeated for 10 cycles. The bimetallic and trimetallic oxide nanocomposites were dried at room temperature and then regenerated by 1 mol/L NaOH solution with a contact time of 20 min. This desorption reagent could efficiently regenerate both of the adsorbents due to the tendency to replace OH particles with particles. The NaOH-regenerated nanocomposites were reused for phosphate removal under optimum conditions. As illustrated in Figure 9, the results showed that with the use of Fe3O4/ZnO, the phosphate removal efficiency had a 2–3% decrease in each cycle and after the 10th cycle, the efficiency declined up to 20%. On the other hand, by using the modified trimetallic oxide nanocomposite, the phosphate adsorption efficiency had less than 1% decrease after each cycle, while, in the 10th cycle, it declined to less than 10%.

Figure 9

Investigating the effect of regeneration for Fe3O4/ZnO and Fe3O4/ZnO/CuO (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C).

Figure 9

Investigating the effect of regeneration for Fe3O4/ZnO and Fe3O4/ZnO/CuO (pH = 3, [Fe3O4/ZnO] = 100 mg/L, , and T = 25 ± 0.5 °C; pH = 4, [Fe3O4/ZnO/CuO] = 100 mg/L, , and T = 25 ± 0.5 °C).

CONCLUSION

In this study, two magnetically separable Fe3O4/ZnO and modified Fe3O4/ZnO/CuO nanocomposites were synthesized and characterized. Then, their ability to adsorb phosphate and the effect of salinity on their performance were investigated and compared. According to the results, the trimetallic oxide nanocomposite had a high capacity to adsorb phosphate in the absence and particularly in the presence of salinity, while Fe3O4/ZnO showed less resistance to salinity and had a lower phosphate removal efficiency. Results demonstrated about 50% increase in phosphate adsorption rate by using Fe3O4/ZnO/CuO and its significant improvement in comparison to Fe3O4/ZnO, due to the decrease in reaction time and energy, and the increase in the phosphate removal efficiency. Regeneration and reusability studies showed that the trimetallic oxide nanocomposite had a notable improvement in comparison to Fe3O4/ZnO due to its negligible adsorption efficiency reduction after each cycle. Moreover, the pseudo-second-order kinetic model perfectly described phosphate removal for both nanocomposites in the presence and absence of salinity.

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

REFERENCES

Bozorgpour
F.
Ramandi
H. F.
Jafari
P.
Samadi
S.
Yazd
S. S.
Aliabadi
M.
2016
Removal of nitrate and phosphate using chitosan/Al2O3/Fe3O4 composite nanofibrous adsorbent: comparison with chitosan/Al2O3/Fe3O4 beads
.
Int. J. Biol. Macromol.
93
,
557
565
.
Eaton
A.
Clesceri
L.
Greenberg
A.
2005
Standard Methods for the Examination of Water and Wastewater
.
American Public Health Association (APHA)
,
Washington, DC, 20001-3710
.
Hasnidawani
J.
Azlina
H.
Norita
H.
Bonnia
N.
Ratim
S.
Ali
E.
2016
Synthesis of ZnO nanostructures using sol-gel method
.
Procedia Chem.
19
,
211
216
.
Hong
R.
Zhang
S.
Di
G.
Li
H.
Zheng
Y.
Ding
J.
Wei
D.
2008
Preparation, characterization and application of Fe3O4/ZnO core/shell magnetic nanoparticles
.
Mater. Res. Bull.
43
,
2457
2468
.
Hong
D.
Yanling
Z.
Qianlin
D.
Junwen
W.
ZHANG
K.
Guangyue
D.
Xianmei
X.
Chuanmin
D.
2017
Efficient removal of phosphate from aqueous solution using novel magnetic nanocomposites with Fe3O4@SiO2 core and mesoporous CeO2 shell
.
J. Rare Earths
35
,
984
994
.
Karthikeyan
P.
Meenakshi
S.
2020
Enhanced removal of phosphate and nitrate ions by a novel ZnFe LDHs-activated carbon composite
.
Sustain. Mater. Tech.
25
,
e00154
.
Kulkarni
S. D.
Kumbar
S. M.
Menon
S. G.
Choudhari
K.
Santhosh
C.
2017
Novel magnetically separable Fe3O4@ ZnO core–shell nanocomposite for UV and visible light photocatalysis
.
Adv. Sci. Lett.
23
,
1724
1729
.
Li
T.
Su
X.
Yu
X.
Han
B.
Zhu
Y.
Zhang
Y.
2018
Preparation of cobalt-containing spinel oxides as novel adsorbents for efficient phosphate removal
.
Environ. Sci. Water Res. Technol.
4
,
1671
1684
.
Lin
D.
Wu
F.
Hu
Y.
Zhang
T.
Liu
C.
Hu
Q.
Hu
Y.
Xue
Z.
Han
H.
Ko
T.-H.
2020
Adsorption of dye by waste black tea powder: parameters, kinetic, equilibrium, and thermodynamic studies
.
J. Chem.
2020
, 5431046.
Luo
W.
Huang
Q.
Zhang
X.
Antwi
P.
Mu
Y.
Zhang
M.
Xing
J.
Chen
H.
Ren
S.
2020
Lanthanum/Gemini surfactant-modified montmorillonite for simultaneous removal of phosphate and nitrate from aqueous solution
.
J. Water Process. Eng.
33
,
101036
.
Nakarmi
A.
Bourdo
S. E.
Ruhl
L.
Kanel
S.
Nadagouda
M.
Alla
P. K.
Pavel
I.
Viswanathan
T.
2020
Benign zinc oxide betaine-modified biochar nanocomposites for phosphate removal from aqueous solutions
.
J. Environ. Manage.
272
,
111048
.
Nezamzadeh-Ejhieh
A.
Hushmandrad
S.
2010
Solar photodecolorization of methylene blue by CuO/X zeolite as a heterogeneous catalyst
.
Appl. Catal. A-Gen.
388
,
149
159
.
Órfão
J.
Silva
A.
Pereira
J.
Barata
S.
Fonseca
I.
Faria
P.
Pereira
M.
2006
Adsorption of a reactive dye on chemically modified activated carbons – influence of pH
.
J. Colloid Interface Sci.
296
,
480
489
.
Stewart
R. H.
2008
Introduction to physical oceanography
.
Texas A & M University, College Station, TX. Open Textbook Library. https://open.umn.edu/opentextbooks/textbooks/introduction-to-physical-oceanography
Taufik
A.
Tju
H.
Saleh
R.
2016
Comparison of catalytic activities for sonocatalytic, photocatalytic and sonophotocatalytic degradation of methylene blue in the presence of magnetic Fe3O4/CuO/ZnO nanocomposites
.
J. Phys. Conf. Ser.
710
,
012004
.
Tju
H.
Taufik
A.
Saleh
R.
2017
Photocatalytic, sonocatalytic, and photosonocatalytic of Fe3O4/CuO/ZnO nanocomposites with addition of 2 different types of carbon
.
AIP Conf. Proc.
1788
,
030131
.
Tran
T.
Nguyen
V.
2014
Copper oxide nanomaterials prepared by solution methods, some properties, and potential applications: a brief review
.
Int. Sch. Res. Not.
2014
,
856592
.
Vakili Tajareh
A.
Ganjidoust
H.
Ayati
B.
2019
Synthesis of TiO2/Fe3O4/MWCNT magnetic and reusable nanocomposite with high photocatalytic performance in the removal of colored combinations from water
.
J. Water Environ. Nanotechnol.
4
,
198
212
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).